1. Field of the Invention
The present invention relates to a capacitive sensor for measuring distance, in particular capacitive sensor for measuring distance to a target in a lithography apparatus.
2. Description of the Related Art
In many applications it is important to measure an electrical current very precisely. For example, charged particle and optical lithography machines and inspection machines for example, typically require highly accurate measurement of the distance from the final lens element of the machine to the surface of a wafer or other target to be exposed or inspected. These machines and others with movable parts often require precise alignment of various parts which may be achieved by measuring distance from the moveable part to a reference point. Capacitive sensors may be used in such applications requiring fine position or distance measurement. When a capacitive sensor is energized, an electrical current flows through the sensor which varies in dependence on the distance between the sensor element and an opposing surface. A precise measurement of this current may be used to accurately determine the measured distance.
To measure an electric current, a measurement circuit may be used having the current to be measured as the input and providing a measurement signal as an output, often in the form of a voltage which may be further processed and converted to a digital signal. There are several factors which contribute to errors in such measurement circuits. These include stray impedance in the input circuitry of the measuring circuit, a limited common mode rejection ratio (CMRR) of the input circuitry, and inaccuracy on the transfer function of the measurement circuit independent of common mode. The value of such stray impedance may change, e.g. depending on factors such as temperature, and disturbances on the input may also change with time. This makes it difficult to compensate for these effects.
It is often necessary to locate the electronic measurement circuits used for driving the capacitive sensors and for generating the desired measurement signals at a distance from the sensors due to the inhospitable environment in which the sensors are located or lack of a suitable place to locate the circuits close to the sensors. In modern lithography applications such as EUV and charged particle systems, the sensors are typically located in a vacuum environment that is very sensitive to contaminants and outside disturbances, and which creates problems with heat removal from electronic circuits if they are located in the vacuum environment, and impedes access for maintenance for such circuits.
The wiring connections between the sensors and remotely located driving and measurement circuits introduce parasitic capacitances into the system which affect the reading of the sensor. If the measuring circuits could be located at the sensor probe, the sensor current could be measured directly and accurately. Because of these parallel parasitic capacitances introduced by the cabling system, measurement of current flow in the sensor is often avoided in systems with remotely located measuring circuits. Conventional solutions introduce measurement errors which need to be taken into account, usually by calibrating the combined sensor and wiring installation. The longer the wiring connection, the more severe these problems become.
The requirement to calibrate the sensors in combination with the sensor wiring reduces flexibility in designing and building sensor systems and increases their cost, and it adds a requirement for recalibration whenever a sensor or its wiring is replaced, making such a replacement complex, time-consuming, and expensive.